1. Field of Invention
The present invention relates generally to a multi-faceted polygon for a laser writer and to a method for creating such a multi-faceted polygon.
2. Background of the Related Art
In laser printing devices, such as laser printers, copiers and similar devices, a laser beam source is directed onto a multi-faceted polygon having a plurality of reflecting surfaces for permitting each surface to reflect the laser beam onto a recording medium, such as film or photographic paper. A circular hole passes through the center of the polygon for receiving a cylindrical shaped motor shaft which rotates the polygon at a constant predetermined angular velocity. As the polygon is rotated within the printing device, each reflecting surface of the polygon successively writes one complete line on the recording medium.
Ideally, each surface of the polygon writes onto the recording medium with a constant exposure level to prevent artifacts due to exposure variations. To provide this constant exposure level, the intensity of the beam and the time it takes for each reflecting surface to write a complete line must remain constant throughout the entire writing process. The beam intensity is relatively easy to maintain at a constant level as is well known in the art. However, if a commercially available polygon is installed into an existing laser writer or if one wishes to assemble their own laser writer using a commercially available polygon, alignment of the polygon to obtain the same writing time for each reflecting surface is time consuming and difficult, as is described in detail below.
This problem arises from the fact that, since each reflecting surface must be maintained at a constant distance from the recording medium, the center of rotation of the polygon must be precisely known. This is difficult because, when the polygon is manufactured, the polygon is placed on a well known and presently utilized grinding apparatus. It is index rotated, with each surface being precisely cut. The result is that each reflecting surface is ground to essentially the same depth. This means that radii drawn perpendicular from the reflecting surfaces, to the center of rotation of the grinding machine, are equal and hereinafter referred to as d.sub.1. After this operation, the center of rotation of the grinding machine is the same as the center of rotation of the polygon. Upon removal from the grinding operation, the finished polygon must be placed on the motor shaft of a scanner system. The motor shaft, whose rotational center is the center of rotation for the scanner system should exactly coincide with the center of rotation of the polygon, so that, as the polygon is rotated, each reflecting surface substantially maintains distance d.sub.1 from the center of the shaft, thus ensuring quality images.
Presently, locating the center of rotation of the polygon when it is subsequently placed on the laser printer shaft is typically done by complex measuring techniques. However, this is time consuming because the reflecting surfaces may not be physically touched because of their unique physical characteristics. This makes necessary non-contact measurements which are both time consuming and costly.
Consequently, a need exists for providing an improved polygon which includes a means for locating the center of rotation of the polygon when it is thereafter placed on the laser scanner shaft.